(a). Field of the Invention
The present invention relates in general to radio frequency (RF) power control, and more particularly to a system and method that adjusts the power supply according to a data rate for transmitting RF signals in good quality.
(b). Description of the Prior Arts
In the Internet era, people tend to consume a lot of information rapidly. And along with the progress of wireless communication technology, people also want to be freed from the limit of wire connections. Wireless local area network (WLAN), combing the network and wireless communication technologies, is then an ideal solution for these needs.
However, in current WLAN applications, a fixed power supply or a fixed bias source is used for RF power amplification. To achieve the goal of real-time communication, a high data rate is needed. However, the transmitting power cannot be retained in a fixed level while the data rate is increasing. When the data rate increases, the transmitting power is lowered, and thus the transmission distance is shortened and in most cases the transmission quality is lowered as well. This is because the average power and peak-to-average power ratio, in the case of the high data rate, are both increased, and the output current and voltage of the power supply need to be upgraded to meet the increased average power and peak-to-average power ratio respectively. Therefore, the fixed power supply or the fixed bias source would cause the power inadequacy of transmitters of the current WLAN applications.
These transmitters commonly employ a power-amplifying unit to amplify RF signals to be transmitted. FIG. 1 is a diagram showing a typical output power-to-input power curve of the power-amplifying unit. The horizontal axis of FIG. 1 is the RF input power while the vertical axis of FIG. 1 is the corresponding RF output power. As shown in FIG. 1, the curve can be divided into a linear region and a saturation region. In the linear region, the output power is varied in direct proportion to the variation of the input power. However, for the same increase of the input power, the increase of the output power in the saturation region is lower than that in the linear region. More critically, the data error rate in the saturation region is higher than that in the linear region. Thus, it is desirable to extend the operating range of the linear region as large as possible and to avoid operating in the saturation region.
When a high data rate is required and a fixed power supply is adopted, the RF input power may be increased to upgrade the RF output power, thereby avoiding the power inadequacy mentioned above. Although the increase of the RF input power could extend the transmission distance, the power-amplifying unit may operate in the saturation region, result in a higher data error rate and thus lower the signal quality. On the other hand, the RF input power or the gain of the power-amplifying unit may be decreased to preserve the signal quality, nevertheless, the output power cannot be reinforced if the provided power is fixed. Besides, the RF output power is conventionally adjusted according to the transmitting signal strength rather than the data rate.
Moreover, if a fixed high power supply is employed, then it will bring power waste when the data rate is low. Many WLAN applications use batteries for power provision, and the power waste would lower the battery duration and then impact the performance of these applications.
In sum, the drawbacks of using a fixed power supply or a fixed bias source for power amplification of RF signal transmission include: (1) the lowered transmitting power and shortened transmission distance, or the lowered data rate and decreased throughput; (2) the degraded signal quality while preserving the transmitting power; and (3) the power waste when the data rate is low.